The production of polymers and copolymers of lower .alpha.-olefins, particularly propylene and ethylene, has gained substantial commercial importance. The polymeric products are relatively inexpensive and exhibit a number of commercially useful properties. In the case of the polymerization of ethylene, the process is relatively uncomplicated in that the product type is not influenced by the manner in which the ethylene molecules add to the growing polymeric chain and the polymer product does not exist in stereoisomeric forms.
In the case of polypropylene, however, the presence of pendant methyl groups on the polymeric chain provides the possibility of several product types depending on the stereoregularity with which the propylene molecules add to the growing chain. Much if not most of the commercial polypropylene is crystalline and results from the stereoregular addition of propylene molecules in a regular head-to-tail manner. The form of polypropylene having a substantial proportion of random addition is termed atactic and this amorphous form of the polymer is less desirable. If present in a significant proportion, the atactic polypropylene must be removed as by an extraction process to provide the more desirable crystalline material. Also significant from a commercial standpoint is the activity of the polymerization catalyst. A number of the early polymerization catalysts, e.g., trivalent titanium, chromium or vanadium, were of relatively low activity and the polymeric product contained significant amounts of catalyst residues. The removal of such residues as by a deashing step was required in order to obtain commercially acceptable properties.
The more recent olefin polymerization catalysts, generally based on titanium and particularly tetravalent titanium, are more stereoregulating and of sufficient activity so that extraction and deashing steps are not required. In terms now employed conventionally, the high activity olefin polymerization catalysts are formed from a solid catalyst component, often termed a procatalyst, which typically contains magnesium, titanium and halide moieties, a cocatalyst which is usually an organoaluminum compound and a selectivity control agent which may be provided as a partial or total complex with the cocatalyst. Although each of these three components has a significant influence on the catalyst and the polymerization process, as well as on the polymeric product so produced, the nature of the catalyst and the polymeric product appear to be most influenced by the procatalyst and much of the research directed toward catalyst improvement has been devoted to solid catalyst components.
Many of the procatalyst species arise from treatment of a magnesium compound with tetravalent titanium halide, often in the presence of a halohydrocarbon and an electron donor. Kioka et al, U.S. Pat. No. 4,330,649, describe a solid catalyst component (procatalyst) obtained by heating a soluble magnesium compound with a higher alcohol in the presence of an ester to produce a solution. To this solution is added titanium tetrachloride and an electron donor to form the procatalyst. Band, U.S. Pat. No. 4,472,521, reacts a magnesium alkoxide wherein each alkoxide moiety has 4 or more carbon atoms in the presence of an aromatic hydrocarbon. Titanium tetrachloride and an electron donor are added to the resulting solution to form a solid procatalyst which is then post-treated with a transition metal halide. Arzoumanides, U.S. Pat. No. 4,540,679, produces an olefin polymerization catalyst component by contacting a suspension of magnesium ethoxide in ethanol with carbon dioxide. The addition of organoaluminum compound in hydrocarbon results in the production of granular particles which are employed as support for a titanium compound upon contact of the particles with titanium tetrachloride. Nestlerode et al, U.S. Pat. No. 4,728,705, solubilize magnesium ethoxide in ethanol with carbon dioxide and spray dry the resulting solution or use the solution to impregnate carrier particles. The solid particles obtained by either modification are contacted with titanium tetrachloride to form a procatalyst.
A somewhat different process is described by Job, U.S. Pat. No. 4,710,428, wherein a magnesium compound of the general formula EQU Mg.sub.4 (OR).sub.6 (ROH).sub.10 A (I)
is formed wherein R independently is lower alkyl of up to 4 carbon atoms and A is at least one anion having a total oxidation state of -2. This magnesium complex is contacted with a tetravalent titanium halide, a halohydrocarbon and an electron donor to form the procatalyst.
More recently, olefin polymerization procatalyst precursors have been produced which contain titanium moleties as well as magnesium moieties. In copending U.S. patent application Ser. No. 599,610, filed Oct. 18, 1990 and now U.S. Pat. No. 5,106,806, there is produced a complex alkoxide compound having the illustrative general stoichiometry of EQU Mg.sub.3 Ti(OR).sub.8 X.sub.2 (II)
wherein R has the previously stated meaning and X is an anion derived from a phenolic compound. Such complex alkoxides are produced from a magnesium alkoxide, a titanium tetraalkoxide and the phenolic compound in alkanolic solvent. Alkanol is removed from a solution of this product in hydrocarbon or halohydrocarbon to provide a clear solution. A solid procatalyst is produced by the addition of a tetravalent titanium halide and an electron donor to the solution. The procatalyst is then converted to a polymerization catalyst by contacting with an organoaluminum compound and a selectivity control agent. This catalyst is an effective high activity olefin polymerization catalyst and provides polymeric product having good properties in high yield (high catalyst productivity). However, it would be of advantage to provide a simplified method of producing such an olefin polymerization catalyst.